Silicon photonic integrated circuits (Si PIC) have been awarded with tremendous attention due to their high density integration capabilities of coupler, modulator, photodiode, avalanche photodiode, polarization diversity components and multimode interferometers. However, link budget deficiency has limited the application of Si PIC. In one aspect, the coupling efficiency of distributed feedback lasers (DFB-LD) and Si PIC has major influence on the link budget.
Up to present time, there are two coupler structures adopted in Si PICs. The first coupler structure is grating coupler, which is favored for its on-chip characterizing compatibilities and high coupling efficiency with less than 2 dB insertion loss. However, one disadvantage of grating coupler is its limited spectral bandwidth, and the typical bandwidth within 1 dB variation is less than 20 nm. Another unobvious but significant disadvantage is that its vertical packaging structure cannot be easily designed into QSFP28 form factors which have already been the main stream packaging form factor in data center applications.
The second coupler structure is edge coupler, usually with an inverse tapered silicon waveguide surrounded by a SiO2 cantilever structure. Edge couplers are in-plane structures which can be fully compatible with QSFP form factors. Edge couplers could also provide less than 2 dB insertion loss to single mode fiber, as well as demonstrating coupling tolerance comparable with single mode fiber and very wide spectral bandwidth supporting O-band and C-band simultaneously. Nevertheless, so far, the coupling loss of a DFB LD is still beyond acceptable, due to the mismatch of DFB-LD mode size and edge coupler mode size. Lenses have been adopted to overcome the mismatch between two modes, as shown in the conventional coupling scheme 500 in FIG. 5, but the reflection from the edge coupler facet has impact on the DFB laser.